Early Life on Earth: The Ancient Fossil Record

Abstract

The evidence for early life and its initial evolution on Earth is linked intimately with the geological evolution of the early Earth. The environment of the early Earth would be considered extreme by modem standards: hot (50-80degreesC), volcanically and hydrothermally active.. anoxic. high UV flux. and a high flux of extraterrestrial impacts. Habitats for life were more limited until continent-building processes resulted in the formation of stable cratons with wide, shallow, continental platforms in the Mid-Late Archaean. Unfortunately there are no records of the first appearance of life and the earliest isotopic indications of the existence of organisms fractionating carbon in similar to3.8 Ga rocks from the Isua greenstone belt in Greenland are tenuous. Well-preserved microfossils and microbial trials (in the form of tabular and domical stromatolites) occur in 3.5-3.3 Ga, Early Archaean, sedimentary formations from the Barberton (South Africa) and Pilbara (Australia) greenstone belts. They document life forms that show a relatively advanced level of evolution. Microfossil morphology includes filamentous, coccoid. rod and vibroid shapes. Colonial microorganisms formed biofilms and microbial mats at the surfaces of volcaniclastic and chemical sediments. some of which created (small) macroscopic microbialites such as stromatolites. Anoxygenic, photosynthesis may already have developed. Carbon. nitrogen and sulphur isotopes ratios are in the range of those for organisms with anaerobic metabolisms, such as methanogenesis, sulphate reduction and photosynthesis. Life was apparently distributed widely in shallow-water to littoral environments, including exposed, evaporitic basins and regions of hydrothermal activity. Biomass in the early Archaean was restricted owing to the limited amount of energy that could be produced by anaerobic metabolisms. Microfossils resembling oxygenic photosyrithesisers. such as cyanobacteria, probably first occurred in the later part of the Mid Archaean (similar to2.9 Ga), concurrent with the tectonic development of suitable shallow shelf environments. The development of an oxygenic metabolism allowed a considerable increase in biomass and increased interaction with the geological environment.